The Lithotripsy process is used for breaking up kidney stones and other stones formed in the body without the need for surgery. Once stone are broken up into small fragments they can leave the body through the renal system. The stones are broken up by focusing an acoustic pulse or shockwave. Early systems used a spark gap as an acoustic energy source. Newer systems use the repulsion of a conductive diaphragm from an energized coil to generate a shock wave in a liquid, typically water, which is focused and coupled into the body. The prior art is shown in FIGS. 1 and 2. A major problem in prior art systems is a lack of an active mechanism to keep the shock wave focused on the stone. When shock wave is not focused on the stone damage to the kidney and other organs can occur. In order to minimize this damage current systems use power levels below optimal. Current systems also spread the size of the beam at the focal point to cover an area larger than the stone, in order not to miss the stone. This further reduces power density and increases tissue damage. At lower power density the stone does not disintegrate completely, causing extra difficulty in passing the fragments through the renal system. The reason the shock wave can not be precisely placed on the stone is the constant movement of the kidney, mainly because of breathing, as well as the fact that the shock waves are not visible to the x-ray or ultrasound equipment used to locate the stone, so the operator has to estimate where the point of maximum shock wave intensity, or focus, is located, without actually being able to see it. Since different tissues have different acoustic velocities the focal point inside the body may be different than the focal point measured in water during calibration. In general there are two types of errors: pointing errors (or x-y errors) and focusing errors (z axis errors). A three dimensional (3D) tracking system can eliminate or reduce x, y and z axis errors.
U.S. Pat. Nos. 4,913,156, 4,915114, 5,031,626, 5,158,085 and 6,942,617 offers a partial solution by mechanically moving the lithotripsy head in response to an ultrasound scanner continuously imaging the stone. Because of the large mass of the lithotripsy equipment the tracking rate possible by mechanical tracking is limited. Also, the main problem is not solved by this patent as the spatial position of the shock wave focus is assumed, not continuously measured and tracked. If there is any calibration error or a displacement caused by tissue, the system will track an imaginary focus which may be offset from the real focus of the shockwave. This risk prevents higher powers from being used, as in the case of an offset the shockwave will always focus on the tissue instead of the stone. A further limitation of any system requiring mechanical steering of the lithotripsy head is that any relative motion between the head and the body affects coupling efficiency and introduces further errors by deforming the tissue or the coupling fluid. By the way of example, when the tissue or coupling fluid inside lithotripsy head is deformed from a parallel shape to a wedge shape the direction of the beam will change. Any air gap introduced during such motion will prevent energy coupling to the tissue.
U.S. Pat. No. 5,658,239 uses the reflected shock wave to locate the stone but relies on mechanical motion of the whole lithotripsy head to correct for placement errors. As before, the large mass of the head prevents effective tracking and changes of the head to tissue interface are undesirable.
U.S. Pat. No. 5,131,392 uses a phased array inside an MRI machine to focus a shock wave, in conjunction with the capability of MRI to locate the stone. Again, the locating mechanism is separate from the shockwave mechanism, allowing an offset error to exist. Furthermore, in an MRI machine very poor acoustic coupling exists between the large spherical machine and the target tissue, not allowing the efficient and localized coupling of a lithotripsy system.
It is desired to have a compact 3D tracking system (tracking both pointing errors and focus errors) which continuously aligns the actual shock wave focus with the stone, regardless of shock wave displacements inside the body. Furthermore, it is desired to track without the use of moving parts, in order to achieve high tracking rate. It is also desired to track without introducing relative motion between the lithotripsy head and the body tissue. It is further desired to focus the shock wave to the smallest practical spot in order to achieve the highest degree of stone disintegration. These and other objects of the invention will become apparent from the drawings and the disclosure.